An industrial robot comprises a plurality of arms that are rotatable relative to each other about a plurality of axes. The movements of the axes are driven by motors mounted on each axis. The speeds and accelerations of the axes are controlled by the control system of the robot that generates control signals to the motors. The control system comprises drives that control the motors by converting direct current to a variable alternating current in dependence on control signals from an axis computer. Each motor has a drive of its own. The control system may have one or more axis computers. Further, the control system comprises a main computer that is adapted to execute a program with instructions for the movements and that supplies the axis computer with control instructions. These control instructions are then transformed by the axis computer into control signals for the drives. The function of the axis computer is thus to ensure that orders from the main computer are carried out. The task of the main computer is to plan the movement path of the robot, so-called path planning, and the task of the axis computer is to ensure that the robot completes the planned path. The control signals to the drives determine motor torque, motor speeds and drive currents for each axis. The main computer also has many other tasks, such as handling I/O systems, application programs, interpolation, and communication with external systems.
A conventional control system for an industrial robot comprises a coherent unit, where the different parts in the control system, such as main computer, axis computer and drives, are mounted in one and the same cabinet with a common power supply, a common interface externally, and a common casing. The control system is provided with an internal bus for communication between the different parts.
From European patent No. 728 559 B1, it is known, instead, to arrange the drives directly on the robot. Each one of the drives is mounted adjacent to the motor that it is to control. This is advantageous since the cables between the drives and the motor will be shorter, which reduces the radiation of high-frequency electromagnetic waves, which in turn may cause disturbance. Another advantage of shorter cables is that the risk of incoming EMC disturbances on the control signals is reduced.
It is common practice to have several robots arranged together in manufacturing cells or along a production line. In such applications, the robots are either provided with individual control systems or with a common control system. Such a common control system comprises a main computer or one or more axis computers as well as one drive unit per robot. A drive unit comprises a plurality of drives, usually between six and nine drives depending on how many motors are to be controlled. The axis computers and the main computer communicate via an internal bus.
One disadvantage of these control systems is that they are inflexible. If it is desired, for example, to add a new function or replace some part of the control system, it is necessary today to intervene and make changes in the existing control system. To be able to add more robots, the existing control system must either be oversized even from the start regarding computer utility and power supply, or the whole of or parts of the control system must be replaced or be rebuilt to obtain the necessary computer utility and power supply.